1. Field of the Invention
The present invention relates to a thin film transistor array. More particularly, the present invention relates to a thin film transistor array having storage capacitors that can be easily repaired.
2. Description of the Related Art
The rapid development of multimedia systems come about as a result of the progress in manufacturing semiconductor devices and monitors. In the past, cathode ray tube (CRT) is the principal display device in the market because of their fine display quality and moderate price. However, due to the bulkiness of CRT and the environmental concerns regarding the production of hazardous radiation and the relatively high power consumption, it has been gradually phased out and replaced by more environmentally friendly display devices with a compact, slim and light body. One such display device is the thin film transistor liquid crystal display (TFT-LCD). Because of the high display quality, superior spatial utilization, low power consumption and radiation free operation characteristics, TFT-LCD has become one of the mainstream products in the market.
A conventional thin film transistor liquid crystal display (TFT-LCD) typically comprises a thin film transistor array, a color-filtering array and a liquid crystal layer. The thin film transistor array comprises an array of thin film transistors and a pixel electrode corresponding to each thin film transistor. Each thin film transistor serves as a switch for switching a liquid crystal display unit. In addition, each pixel unit is selected through a scan line and a data line. By applying a suitable operating voltage to the selected scan line and data line, a pixel data is displayed on the pixel unit. Furthermore, a portion of the pixel electrode will cover the scan line or a common line to form a storage capacitor. In the conventional technique, the most common storage capacitor can be categorized into the metal-insulator-metal (MIM) type and the metal-insulator-indium tin oxide (MII) type. The following is a more detailed description of these two types of storage capacitor structures.
FIG. 1 is a schematic cross-sectional view of a conventional metal-insulator-metal (MIM) type storage capacitor. As shown in FIG. 1, a storage capacitor having an MIM structure is formed through coupling a scan line or common line 100 with an upper electrode 120 on top. It should be noted that the scan line or the common line 100 is electrically isolated from the upper electrode 120 through a gate insulation layer 110. Hence, the capacitance (Cst) of the storage capacitor is related to the thickness of the gate insulation layer 110. In other words, the smaller the thickness of the gate insulation layer 110, the larger will be the capacitance (Cst) of the storage capacitor. In addition the pixel electrode 140 is electrically connected to the upper electrode 120 through a contact 132 in the passivation layer 130.
FIG. 2 is a schematic cross-sectional view of a conventional metal-insulator-ITO (MII) type storage capacitor. As shown in FIG. 2, a storage capacitor having an MII structure is formed through coupling a scan line or common line 200 with a pixel electrode 230 on top. One major difference from the MIM structure is that the scan line or common line 200 and the pixel electrode 230 in the storage capacitor having the MII structure are electrically insulated through a gate insulation layer 210 and a passivation layer 220. Hence, the capacitance (Cst) of the storage capacitor is related to the thickness of the gate insulation layer 210 and the passivation layer 220. In other words, the smaller the thickness of the gate insulation layer 210 and the passivation layer 220, the larger will be the capacitance of the storage capacitor (Cst).
In general, the capacitance (Cst) of a storage capacitor having the MIM structure is larger than the capacitance (Cst) of a storage capacitor having the MII structure. The principle reason is that the two metallic layers in the MIM structure are isolated from each other by a single gate insulation layer 110 while the two metallic layers in the MII structure are isolated from each other by a gate insulation layer 210 and a passivation layer 220.
Because the storage capacitor inside the pixel structures is used for retaining the voltage applied to various pixel units of a thin film transistor liquid crystal display (TFT-LCD), the display panel will have a better display quality when the capacitance (Cst) of the storage capacitor is large. Therefore, conventional TFT-LCD frequently deploys storage capacitors having a MIM structure.
Although a storage capacitor having a MIM structure has a larger capacitance, the production process often produces layers having defects such as micro-particle inclusions or voids that can impair the storage capacitor and produce defective dots.